Why healthy ecosystems matter: the case of mangroves in the Mekong Delta

14 December 2011 | Article

If protected, nature has an amazing ability to recover after disturbance. But the ability to recover depends on the health and resilience of the ecosystem. Mangroves in the Mekong Delta illustrate this principle.

On a recent field trip, the Building Coastal Resilience (BCR) project team visited several mangrove forests and saw the threats they face. In many areas, mangroves have been cleared to make shrimp ponds, often leaving a very thin strip of mangrove forest behind. Where mangroves remain, they are mostly single-species plantations, with very low biodiversity. In Soc Trang, for example, most of the mangroves are composed of Sonneratia while in Can Gio, Rhizophora dominate. A forest manager at Soc Trang explained that it was much more convenient from to just plant one species. The result, as pointed out by a government researcher at Can Gio where disease and pests are common, is that the health of the mangrove forest is much reduced.

Single-species mangrove plantations may also be more susceptible to coastal erosion. During our field visits, we saw areas of rapid coastal erosion where government officials blamed the mangrove loss and dyke collapse on sea level rise and stronger wave action. At these sites, the mangroves consisted of a thin strip, no more than 100 meters wide, located right next to the sea dyke (see picture). In Vam Ray village in Kien Giang Province, the village leader, when asked if the mangrove loss was due to overexploitation by local people, said no and that sea level rise and wave action were to blame. Neither the government officials nor the village leader questioned the position of the dyke or its impact on the mangroves. If the dyke was not in place, a mangrove forest with a diversity of species able to adapt to new conditions could survive by retreating inland. Over long time periods, ocean currents rather than sea level rise plays an important role in determining which areas experience high erosion and mangrove loss, and this has not been taken into account at most of the sites we visited.

The poor health and the presence of physical barriers make the delta’s mangroves very vulnerable to disturbance. Or to put it another way, the “adaptive capacity” of the delta’s mangroves is very low. This raises the following “chicken and egg” question: Did the loss of the mangroves result in increased coastal erosion or did increased coastal erosion (perhaps as a result of changes in ocean current) result in loss of the mangroves? Both may be true: coastlines protected by healthy mangroves are resilient to wave action and erosion, but once these are degraded beyond a certain point, their protective capacity collapses and the coast is exposed to wave action that prevents regeneration.